One balmy spring evening three years ago, Barbara Cook surveyed a scant turnout of ringside worshippers at the Cafe Carlyle and proclaimed us all "small but mighty." We have been, too, because the show we elicited from her proved unimprovable. Perfect!

One balmy spring evening three years ago, Barbara Cook surveyed a scant turnout of ringside worshippers at the Cafe Carlyle and proclaimed us all "small but mighty." We have been, too, because the show we elicited from her proved unimprovable. Perfect!

Truth to tell, there's not much difference — other than the head count — between Cook giving her all to a late-night, weekday cabaret clique and her giving her all to the SRO crowds that have been flocking to her Mostly Sondheim concerts on Sunday and Monday nights this summer at the Vivian Beaumont Theater in Lincoln Center. All really is all in this case, and, as Cook cooks it, it's everything. Darned if you don't feel you've got her all to yourself alone.

"Can we start dispensing superlatives right now?" proposed AP critic Michael Kuchwara in his review of her act. "Musical theatre doesn't get much better than this."

That's right. Barbara Cook, an icon of the genre for 51 years, is still doing musical theatre whether she is doing it in night clubs or at Carnegie Hall. The girl can't help it.

Like Elaine Stritch, another septuagenarian particularly adept at audience contact and Sondheim content, she's enjoying one of Broadway's great third acts — and, for this latest triumph, she took her song(s) cue from Frank Rich, who was among the "small but mighty" who huddled around her that spring night in 1999. Rich penned a piece for The New York Times Magazine celebrating Stephen Sondheim's 70th birthday and asked the composer about the songs that he had written as well as the songs that he wished he had written. The article caught the eye of Wally Harper, Cook's accompanist-arranger-conductor for the past 29 years, and he envisioned a show in that.

Cook concurred. At the time she was easing herself into the heady waters of Sondheim for a new act at Feinstein's at The Regency, and Rich's interview pointed the way to immerse herself even more by punctuating a steady stream of Sondheim with songs he admired. It's slightly surprising that the singer and the songs never truly got together, given the way she emotes a lyric and the way he writes three-act plays in song form, but their musical paths intersected only once — in Lincoln Center's star-stacked 1985 concert version of Follies — in which Cook acted the living daylights out of "In Buddy's Eyes."

"When I first started out," she recalls, "I didn't give much thought to acting a song. That evolved. Now I think of it as living inside a song and singing my way out — inhabiting it, feeling it, making it felt from my core to your core. That's the only way I can explain it."

Cook from core to core conveys more, communicates better, than she did as an ingenue — a better singer because she is a better actress — and Sondheim gives her a chance to flex this muscle in spades. "Steve has been wonderfully supportive — so kind and complimentary — about this show," she says. "He came up to me the night he saw it and said, 'Do you know what song moved me the most? It wasn't one of mine. It was 'San Francisco.' I don't know why, but tears just started streaming down my face.'"

The song that moves Cook the most is Sondheim's first love song, "So Many People," from his first (and, paradoxically, most recently produced "new") show, Saturday Night. At the time it was written — 1955 — Cook was lighting up Broadway as its resident ingenue in Plain and Fancy,for which she won a Theatre World Award as a Promising Personality of the 1954-1955 season. She had arrived here four years earlier and, at 23, fresh off the bus from Atlanta, had landed a lead in Flahooley. Sammy Fain and E.Y. "Yip" Harburg threw her a couple of songs — "He's Only Wonderful" and "Here's to Your llusions" — which shrewdly upstaged the four-octave warblings of the show's nominal star, Yma Sumac.

Wherever she landed after that, she seemed to find an archetypal ingenue role inevitably accompanied by a number that she'd introduce and straightaway turn into a signature song. Leonard Bernstein is said to have shaped "Glitter and Be Gay" around her voice and phrasing when she was Cunegonde in Candide. And, as the amorously inclined pen pal of She Loves Me, she got the romantic message with an exquisite serving of Sheldon & Jerry's (Harnick and Bock's) "Ice Cream." (It being a Sondheim favorite, she reprises it in Mostly Sondheim untransposed — but approaches the high notes more cautiously.) When Meredith Willson's "Till There Was You" entered our consciousness, it was via her everlasting lilt, playing a Tony-winning Marian the Librarian to Robert Preston's Tony-winning Music Man. "He was an incredibly consistent performer and a joy to work with," she says of Preston. "He brought such energy to the show. I think that's why it ran as long as it did."

The show she thinks should have run longer than it did was a far-too-fleeting revival of The King and I at City Center, opposite Farley Granger. "We were both frightened to death because we were following legends in those roles, but I think it came off beautifully. I'd really have loved to have played that show for a long time." She's also remembered fondly for a couple of other Rodgers and Hammerstein roles (Ado Annie in Oklahoma!and Carrie in Carousel) and one from Kern and Hammerstein (Magnolia in Show Boat).

Every once in a while, she has accepted a straight dramatic role (Patsy in the original Little Murders, for instance). The last time she did that was 30 years ago on the very stage where she is now working — for a play called Enemies with Frances Sternhagen and Nancy Marchand — and she can't resist regurgitating her nasty John Simon notice, something to the effect that now that Barbara Cook's voice is fading she's taking up acting. Rarely has a critical call been so off the mark. Her voice and her acting never deserted her; now they're in the league with vintage wine. The temptation must have been great for Cook to turn up the heat on Simon's feet, but she opts to take the higher road. Being right and surviving in great shape are their own rewards, after all.